We report here on a low-cost, optical oxygen sensor as an attractive alternative to the widely used amperometric Clark-type oxygen electrode for measuring dissolved oxygen tensions in cell cultures and bioreactors. Our sensor is based on the differential quenching of the fluorescence lifetime of a chromophore in response to the partial pressure of oxygen. This is measured as a phase shift in fluorescence emission from the chromophore due to oxygen quenching when excited by an intensity modulated beam of light. In this report we demonstrate the advantages of lifetime-based optical methods over both intensity based optical methods and amperometric electrodes. Our sensor is particularly suitable for measuring dissolved oxygen in bioreactors. It is autoclavable, free of maintenance requirements and solves the problems of long term stability, calibration drifts and reliable measurement of low oxygen tensions in dense microbial cultures that limit the utility of Clark-type electrodes.